Sigma Tags

These tags are written by Sigma/Foveon cameras. In the early days Sigma was
a class leader by releasing their maker note specification to the public,
but since then they have deviated from this standard and newer camera models
are less than consistent about their metadata formats.

Sigma LensType values are hexadecimal numbers stored as a string (without
the leading "0x"). Decimal values have been added to differentiate lenses
which would otherwise have the same LensType, and are used by the Composite
LensID tag when attempting to identify the specific lens model.